The present invention relates to a mobile terminal for a wireless telecommunication system with an accurate real time generation and a method for providing an accurate real time information in a mobile terminal for a wireless telecommunication system. Particularly, the present invention relates to a mobile terminal comprising a real time means providing real time information on the basis of a low frequency oscillation signal.
Mobile terminals for wireless telecommunication systems comprising real time means providing real time information on the basis of a low frequency oscillation signal are known in many different variations. For example, real time means can be quartz devices operating in the low frequency range of e.g. several kHz. Such devices usually have an accuracy of only 50 ppm, which leads to an inaccuracy of 130 seconds per month. On the other hand, these quartz devices are cheap and consume very little energy compared to highly accurate time base systems. The high inaccuracy, however, is not acceptable for consumer applications. In order to improve the accuracy normally either selected quartz devices are used or the frequency offset is measured during the manufacturing process and stored in the system. During normal operation this offset is used to compensate the frequency error.
Both above-mentioned approaches are cost-intensive. The use of selected quartz devices raises the cost of the parts of the mobile terminal, whereas measuring the offset during the manufacturing process requires additional test equipment and test time during the manufacturing. Further, both solutions are not able to cope with the aging of the real time means over time, i.e. the changement of the frequency the quartz device due to aging of the material.
U.S. Pat. No. 5,274,545 proposes a device and a method for providing accurate time and/or frequency information. A low frequency oscillator unit provides real time information which is corrected by means of an external high precision clock signal. The external clock signal is thereby used to update the real time information provided by the internal oscillator unit by means of a feedback loop.
EP 0 726 508 A1 discloses a real time clock for a mobile telephone. The real time clock signals are adjusted in response to a calibration value, whereby the calibration value is calculated depending on external clock signals received from a base station. The calibration value is thereby used to accelerate or slow down the real time clock in order to provide adjusted and accurate real time values.
EP 0 683 443 A2 discloses an electric clock comprising a usual oscillator, as e.g. a quartz oscillator, and a more accurate oscillator. The more accurate oscillator is used as a reference, to which the frequency of the usual oscillator is compared, in order to correct the oscillation frequency of the usual oscillator and to provide an accurate real time information.
EP 0 586 256 A2 discloses a time measurement system in which a slower clock oscillator and a faster clock oscillator are compared to measure the momentary error of the slower clock oscillator. Thereby, the slower clock oscillator and the faster clock oscillator are selectively coupled to a counter means to count the pulses thereof in order to provide an accurate time output.
All above-mentioned time systems are complicated and cost-intensive and therefore not suitable for application in a mobile terminal of a wireless telecommunication system. Further, the above systems are not able to cope with the aging problem in an efficient way.
The object of the present invention is therefore to provide a mobile terminal for a wireless telecommunication system and a method for providing accurate real time information in a mobile terminal of a wireless telecommunication system, in which real time information on the basis of a low frequency oscillation signal within said mobile terminal can be provided in a simple and cost-effective way, whereby particularly the aging problem can be coped with effectively.
The above object is achieved by a mobile terminal for a wireless telecommunication system comprising input means for inputting basic time information, memory means for storing basic time information input via said input means, real time means for continuously providing real time information on the basis of a low frequency oscillation signal and said stored basic time information, processing means for computing a correction value for correcting said real time information to obtain an accurate real time information on the basis of an accurate time difference between a first accurate time information and a second accurate time information and a real time difference between the first real time information and a second real time information from said real time means.
The above object is further achieved by a method for providing accurate real time information in a mobile terminal in a wireless terminal communication system comprising the steps of inputting basic time information, storing said basic time information, continuously providing real time information on the basis of a low frequency oscillation signal and said stored basic time information, and computing a correction value for correcting said real time information on the basis of an accurate time difference between a first and a second accurate time information and a real time difference between a first and a second real time information to obtain accurate real time information.
According to the present invention, a cheap and simple real time means can be implemented in a mobile terminal, whereby the inaccuracy of such a real time means can be corrected on the basis of accurate time information. Thereby, accurate real time information in a mobile terminal of a wireless telecommunication system can be provided in a simple and cost effective way. Further, the computing of a correction value for correcting the real time information to obtain an accurate real time information can be performed any time, so that particularly the aging problem can be coped with effectively.
Advantageously, the processing means of the mobile terminal calculates the correction value on the basis of a difference between the accurate time difference and the real time difference. Thereby, real time information with a high accuracy can be provided within the mobile terminal.
Further advantageously, the first and the second accurate time information are relative time information, i.e. information related and direct proportional to absolute time information. In this way, the correction of the real time information can be achieved in a simple and cost effective way since only relative accurate time information is used for computing the correction value. In this case, the mobile terminal advantageously comprises a receiving means for receiving the, first and the second accurate time information via the wireless telecommunication system. The first and the second accurate time information may for example be the frame number of transmitted GSM frames or the like. The receiving means adapted for receiving the first and the second accurate time information may be the normal receiving means of the mobile terminal for receiving control and user data in the wireless telecommunication system. Alternatively, the mobile terminal may advantageously comprise time reference means for providing the, first and the second accurate time information. Thereby, the mobile terminal may comprise a separate internal time reference means which provides a high precision time base on the basis of a high frequency oscillation signal, for example in the MHz frequency range. Normally, the high precision time base is more energy consuming than a low frequency time base, such as the real time means of the mobile terminal, so that it should not be operated continuously in the mobile terminal. Further, such a high precision time base usually does not comprise a back-up battery so that in case of power interruptions the high precision time base stops to operate.
According to a further aspect of the present invention, the second accurate time information may be input via the input means, whereby the processing means uses the basic time information as the first accurate time information for computing the correction value. The input means may in this case be the normal key pad of the mobile terminal or a separate input means especially adapted for this purpose.
In all above-mentioned aspects of the present invention, the computing means of the mobile terminal may compute the correction value only when the accurate time difference and/or the real time difference exceed a predetermined threshold value. In this case it can be avoided that the elapsed time is too short to enable a computing of an accurate correction value, e.g. when the mobile terminal is switched off, the connection is lost, or the user changes the actual time within a short period.
Further advantageously, the computing means computes the correction value automatically when a predetermined time period has elapsed since the last computation of a correction value. Hereby it can be avoided that the correction value used for correcting the real time value in the mobile terminal is not adjusted within a preset time period. It may be further advantageous if the computing means computes the correction value upon receiving a correction initialisation information. This correction initialisation information may e.g. be input by a user via the input means. Alternatively, the correction initialisation information might be received from a base station through the wireless telecommunication system. Thereby, the real time value used in the mobile terminal may be adjusted to changing time zones or summer time/winter time changes automatically.